A need exists in various fields of human endeavor for an oscillator arrangement that can be rapidly tuned to any desired frequency within its operating range with a high degree of accuracy that is to be maintained for as long as required. This need is particularly pronounced in the telecommunications field, especially in the cellular and cordless telephone system areas where frequency lock between the transmitter and the receiver, or accurate tuning of both transmitter and receiver to the very same frequency which more often than not differs from one communication event to the next, is crucial for establishing quality transmission between such telecommunications devices. Therefore, the transmitter and the receiver of such systems must be equipped with oscillator arrangements that are either very stable, or capable of changing their frequency and/or phase in unison, and preferably both. Such synchronization is becoming more and more critical in view of the current trends of increased transmission rates, reduced spacings between adjacent carrier frequencies, and the use of signal multiplexing, data compression and/or other techniques in an attempt to satisfy the ever-increasing demand for services despite the limited nature of the available transmission spectrum.
An approach that is currently being used for controlling the output frequency of a variable frequency oscillator, such as a voltage controlled oscillator, in a phase-locked relationship to an oscillating input (reference) signal is to interpose an integer divider into a feedback loop between the oscillator output and a phase detector that also receives the input signal and issues a control or error signal indicative or discrepancies between the phases of the two signals. This error signal is then supplied, after being processed in a loop filter and amplifier, to a control input of the oscillator to control the operation (i.e. the oscillation frequency) of the oscillator. The phase detector operates on the edges of its input signals--that is, it detects the lead/lag relationship between such edges and issues a corresponding error signal. As a result, the oscillator output frequency is maintained at a level N times the input reference signal frequency F.sub.r, where N is the integer by which the integer divider divides the output signal of the oscillator.
Experience with oscillator arrangements of this type has shown that some of the signal at the incoming signal frequency F.sub.r leaks through the oscillator arrangement to its output. This F.sub.r leakage produces modulation sidebands in the output signal of the voltage controlled oscillator. To eliminate or at least minimize interference or crosstalk between communication channels, most cellular/cordless telephone systems require that the power carried by such sidebands, commonly called reference sidebands, be at least 50 to 60 dB below that of the carrier. This typically requires that the loop bandwidth F.sub.bw of the oscillator arrangement or synthesizer be maintained at less than about one-tenth of the incoming signal frequency--i.e. below 0.1 F.sub.r. Now, in this particular construction, the frequency step size F.sub.step at the output of the oscillator is also equal to F.sub.r, so that the ratio of the loop bandwidth F.sub.bw and the frequency step size F.sub.step typically amounts to 0.1. Increasing the order of the loop filter in the synthesizer would permit an increase in the loop bandwidth F.sub.bw without degrading the reference sideband suppression; however, this would also reduce the phase margin of the loop and would offer, at best, a relatively small improvement (e.g. a factor of 2 or 3). Consequently, any significant increase in the F.sub.bw :F.sub.step ratio would, in accordance with the prior art, require the use of multiple phase locked loops. Obviously, that would be a very cumbersome and expensive proposition to adopt.